Some recent vehicles are equipped with a smart ignition system that enables engine to start in response to an ID signal wirelessly transmitted from mobile equipment. With the smart ignition system, when the user operates an engine start/stop switch mounted on an in-vehicle instrument panel, a vehicle authentication device transmits a request signal to the mobile equipment. Upon receipt of the request signal, the mobile equipment transmits an own ID signal to the vehicle authentication device. The vehicle authentication device compares an ID code included in the received ID signal with a previously stored ID code, and if both the ID codes match with each other, authenticates the ID code, and enables the engine to start to activate the engine by operating the engine start/stop switch.
Some smart ignition systems have a transponder communication function of communicating with the vehicle authentication device without requiring a power source such as a battery in the mobile equipment, as an auxiliary communication means used when the battery or the like in the mobile equipment becomes exhausted, leading to an uncommunicable state. In the transponder communication function, when the mobile equipment receives starting radio waves transmitted from a coil antenna in the vehicle authentication device, a coil part of a transponder in the mobile equipment generates an electromotive force, and an ID signal is transmitted to the vehicle authentication device by the electromotive force to achieve communication between the vehicle authentication device and the mobile equipment.
The use of the transponder communication function requires the user to operate the engine start/stop switch in the state where the mobile equipment is located near the coil antenna of the vehicle. In order to facilitate the user to recognize the location of the coil antenna, the coil antenna is disposed in the vicinity of the engine start/stop switch.
Regarding the arrangement of the coil antenna, for example, Japanese Patent Publication No. 4705317 proposes that a coil antenna 115 is disposed on the radial outer side of an operation knob 103 of a switch device (engine start/stop switch) 101 as shown in a cutaway side view of FIG. 8. Japanese Unexamined Patent Publication No. 2011-525455 proposes that a coil antenna 215 is disposed along the inner edge of an operation knob 203 as shown in a side sectional view of FIG. 9.